The enzyme adipose tissue lipoprotein lipase (AT-LPL) is rate limiting for the uptake and storage of triglyceride by adipose tissure and has been shown to change in a counter-regulatory manner in obese subjects who lose and then regain weight. The major aim of this protocol is to determine whether these changes in enzyme activity are unique to obese subjects, or represent a more general response to changes in fat mass or fat cell size. To accomplish this, heparin elutable AT-LPL activity will be prospectively measured in obese and normal weight males undergoing weight loss by dieting. Recent studies have suggested that an exercise training mimics many of the metabolic changes seen with weight loss. It is not entirely clear, however, if this training effect is due to small changes in fat mass or fat cell size. For this reason, the effect of 3 months of aerobic training on AT-LPL will also be prospectively evaluated in lean and obese subjects. Careful measurements of changes in fat mass and fat cell size with exercise will help determine to what degree exercise effects can be accounted for by changes in adiposity. High density lipoprotein cholesterol (HDL-C) is correlated to AT-LPL activity and both are increased after either weight loss of aerobic training. By prospectively measuring HDL-C in obese and normal weight subjects before and after either dieting alone or training alone, this study will help delineate the relative importance of exercise versus reduction in adiposity in increasing the concentration of this important antiathrogenic lipoprotein.